[visualradio]

Before substantially modifying the language, make sure your parser can still load vanilla C header files with no new features.

This way if you accidentally create a new language, programmers will still be able to load existing C header files without having to manually write and maintain FFI bindings.

[tomcam]

Adding multiple return values creates a new language

[visualradio]

The more relevant issue is whether or not it preserves backwards compatibility. It is possible for their compiler to continue to consume existing ANSI C and C99 header files and source code, without requiring the programmer to manually write FFI bindings and wrappers to call regular C functions as is the case in many other high-level languages, and also without adding as many features as C++.